Updated: Aug 31, 2020
Authored by David Walker
A look into why the newest variant of the world’s best selling aircraft crashed twice and the Technological failures that led to such disasters.
Air travel is astoundingly and extraordinarily safe. In many ways it may actually be one of the lowest risk activities we do in our entire lives with 1 fatality per 287 million passengers flown in the UK , meaning you’re more likely to be struck and killed by lightning many times over then die in a plane crash. However, accidents, whilst freak in nature and extremely rare, do still happen. With 39.4 million flights daily as of this year , it is merely a matter of probability that some incidents occur.
The incredible safety record of air travel makes the crashes of Lion Air Flight 610 and Ethiopian Airlines Flight 302 of last year even more jarring, especially given the fact they were both using the same type of brand new aircraft. The new iteration of the Boeing 737, the MAX family, was built and supposedly improved upon a design that has been around for over 50 years, and is the basis for the most successful aircraft of both the 20th and the 21st century . If you have ever flown on a short haul flight anywhere in the world (especially if you have flown with Ryanair), the odds are extremely high that you have flown on some variant of the Boeing 737.
However, despite its extreme success and a previously flawless safety record, careless integration of new technology with an old design was unfortunately the leading cause of both incidents involving the 737 MAX.
Figure 1 : ET-AVJ – The Boeing 737 MAX 8 that was involved in the crash of Ethiopian Airlines Flight 302.
A Design meant for Small Engines
The first variant of the Boeing 737 held its maiden flight on April 1967 and has undergone a total of 9 iterations before the production of the 737 MAX. Unsurprisingly this makes it the oldest aircraft to still have a continued present day production line on the planet. To put this in perspective, some of Boeing’s other Aircraft families, the 747,757 and 767, have all been discontinued despite being released decades later, and Airbus’s oldest aircraft family (the A300) was discontinued 12 years ago! Due to the limited technology present in the 1960s and specifications that were applicable 50 years ago, modern day 737s still have a plethora of features that seem extremely out-dated by todays standards. The most notable of these is the extremely low height of the cabin above the ground.
Prior to the 1980s, aircraft engines were somewhat dissimilar to those found on modern airliners. Looking at Figure 2, one can see that the size and overall shape of Turbojet engines found during this time were far smaller than engines today and as a result the aircraft could be designed so that the cabin height was low to the ground. So why keep the aircraft so low? The reason lies in the fact that air bridges, which are commonplace, now, were extremely rare during the 60s and 70s, as they had only just been invented . Furthermore, whilst detachable airport stairs (still used at some UK airports) did exist, they were extremely expensive to use. In order to cut costs for airlines (and make the aircraft more marketable), Boeing built retractable stairs into the cabin of the 737 . This has actually proven to be very successful, with airlines such as Ryanair still using the retractable stairs on their newer 737s today.
Figure 2: An extremely old Lufthansa Boeing 737-200. Take note of the small Turbojet Engines and the retractable stairs built into the cabin.
The issue with this design, however, was that no one in the 1960s could predict how Engine technology would evolve and the issues the cabin height would cause. Simplifying to a great extent; newer, larger Turbofan engines with high bypass ratios are vastly more efficient than older Turbojets due to the fact that moving larger masses of air at a lower speed uses significantly less energy than the other way round (the technical, less simplified reason is long enough to have its own article!). Consequently, Engines on the 737 have been growing in size ever since the aircraft was first introduced, with the 737 Max having some of the largest Engines for any single aisled airliner .
Big Engines and Aircraft Instability
In order for such large new Engines on the 737 Max to fit, they were mounted further forward in front of the wing (rather than underneath), with a flattened base inlet. Due to the huge inlet radius, this was still unfortunately not enough and the whole aircraft had to be raised from the ground by the minimally safe amount to give suitable clearance . Regrettably, for Boeing, their Engineers did not predict the huge amount of instability having such large Engines at a distance so far forward on the airframe would cause (see Figure 3).
Figure 3 : – A head on view showing the size difference of the engines on the new 737 Max compared with the last generation of 737s. Also note the flattened base of the engine inlet.
So you’re probably wondering, why not raise the aircraft even higher so that the new Engines could fit under the wing? The issue with this lies in the fact that every change to an aircrafts airframe design is prohibitively expensive for aircraft manufacturers, as it costs both money and a huge amount of time in research and development. This is the reason why even modern Boeing 737s built today still have paper checklists rather than electronic ones and still have the same window and door design as 50 years ago! If a design change does not cause significant improvements in efficiency and cost, it simply is not worth it.
Unfortunately, the effect of the new Engines on the aerodynamics of the airframe proved to be disastrous. Looking at Figure 6, you can see that when an aircraft is climbing, the rate and success of the climb depends on how and where the moments of the aircraft are acting in relation to the centre of mass. We can see that in order to maintain the angle and direction of the plane (i.e. keep the moments balanced from the centre of gravity), the vertical moment of Thrust at distance x plus the vertical moment of the rear elevator force R at distance y from must be equal to the vertical moment of Lift L at distance z. Mathematically we can write this as:
Figure 4 : A free body diagram of an aircraft climbing at an angle θ. Note that this is a drawing of a Spitfire, not a 737!
By mounting the aircraft engines further forward, we reduce the distance (x) of the engines from the centre of mass. At the same angle, our equation now becomes an inequality (as x is reduced):
Assuming that we maintain the same Thrust, Lift and Elevator settings, the plane will naturally move to a balanced position via a resultant clockwise moment. The only way this will occur however is if the angle θ increases. As you can see from the above equation, this will increase the moment created by the Engines Thrust allowing the moments to be balanced again.
We therefore come to main engineering design flaw of the Boeing 737 Max: the angle θ would increase when the plane was climbing and thus the plane would continuously pitch up, making the plane unstable. The major issue with this is that if the thrust was low enough, it would substantially increase the risk of stalling at thrust levels where stalling should not occur! To combat this issue, Boeing created the software system MCAS that would use sensors to determine when the aircraft was pitching up and electronically control the rear elevators to pitch the plane down (thus increasing the force R in our equation).
As any STEM student will tell you, the issue with software-hardware integration is that it only takes one bug to completely ruin the system. Therefore, it is a necessity for any control system to have multiple inputs and redundancy to ensure reliability. The problem with the 737 Max was that this idea was not followed. A single faulty sensor on both Lion Air Flight 610 and Ethiopian Airlines Flight 302 caused the MCAS system to pitch the plane down violently even when the aircraft’s pitch was low . Unfortunately, this caused both planes to go into periodic nosedives soon after takeoff, which thus lead to the two crashes.
It is a shame that the horrific nature of air crashes and their news coverage often means people don’t truly grasp how safe travelling by air really is. For incidents like the 737 Max crashes to occur, a cascade of faults, poor decisions and some really bad luck over many years has to happen. It is simply unfortunate and bad timing that the result affected so many people. This is not to say however that there are not people at Boeing who should be held accountable, as glaring shortcomings in the 737’s design that were overlooked out of greed and negligence contributed heavily to the crashes.
Perhaps this is a reminder that the technology that Scientists and Engineers work on should and often does have a massive impact on many people, for better or worse. When it doesn’t work, the results can range from irritating to catastrophic, as seen in this article. When it does work, it can greatly improve peoples’ quality of life.... and then everyone starts taking it for granted.
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